A suicide bomber driving an ambulance packed with explosives crashed through the front gate of an Iraqi guard force headquarters today, killing at least seven people and destroying a building, officials said.

It follows a suicide bombing yesterday that killed 65 people in a crowd of police recruits and shattered a relative calm that had lasted for weeks across Iraq. Both attacks were on familiar targets. Militants frequently hit Iraq's security forces to try to keep the country off balance as US troops prepare to withdraw fully at the end of this year.

In today's attack in the eastern city of Baquba, the ambulance broke through the front gate of the Facilities Protection Service compound, which houses the local headquarters and some training grounds for the Iraqi security force tasked with guarding government buildings.

The vehicle exploded when guards opened fire to try to stop it, said Major Ghalib al-Karkhi, the police spokesman for the surrounding Diyala province. Seven people were killed and 74 wounded, he said.

Duleir Hassan, a Diyala provincial councillor who oversees security issues, said it was not clear how the attacker obtained the vehicle.

Baquba is 35 miles north-east of Baghdad.

Yesterday's attack took place on the outskirts of Tikrit, Saddam Hussein's hometown in north-central Iraq.

Dr Raied al-Ani, director of Tikrit hospital, put the death toll in the bombing at 65, with at least 150 wounded. Local officials declared three days of mourning.

Hundreds of recruits had gathered outside the targeted police station to be among the first applicants for 2,000 newly created interior ministry jobs. Authorities said the suicide bomber joined the crowd, then detonated his explosive vest.

Police found an unexploded grenade at the scene, indicating the bomber was using other weapons to maximise the blast. Tikrit is 80 miles north of Baghdad.

No group has claimed responsibility, but the attack was in the style of al-Qaida-linked Sunni groups that want to keep fellow Sunnis from joining the security forces.

A statement posted on a militant website by the Islamic State of Iraq, an al-Qaida front group, praised the bombing as a "suicide martyrdom" operation.

The Tikrit bombing infuriated Iraqi officials including the prime minister, Nouri al-Maliki, who vowed to strengthen the security forces.